Lubricating composition comprising glycerol monoethers

ABSTRACT

Disclosed is a lubricating composition including: at least one base oil; and at least one glycerol monoether characterised in that one of the alcohol functions of the glycerol forms an ether function with a linear or branched, alkyl or alkylene R group including 6 or 7 carbon atoms, the composition being an engine lubricant.

The invention relates to the field of friction modifiers, and, inparticular, organic friction modifiers. More particularly, the inventionrelates to a lubricating composition comprising a friction modifierselected from glycerol monoethers. The invention also relates to the useof this lubricating composition for lubricating an engine.

One of the major objectives of the present automotive industry is thereduction of the fuel consumption of engines, especially motor vehicleengines, and thus the improvement of fuel economy vehicles. Reducingfriction in an engine is an effective way to achieve fuel economy. Thus,much research has been done on friction modifiers.

Four major groups stand out among friction modifiers: nanoparticles,polymers, organomolybdenum compounds and organic molecules.

Although nanoparticles and polymers are little used today, this is notthe case for organomolybdenum compounds, which represent the mostimportant family of friction modifiers. The best-known and most widelyused inorganic friction modifier is molybdenum dithiocarbamate (MoDTC).These inorganic friction modifiers, although very effective, havecertain drawbacks. In fact, they may induce some fouling and corrosionof engine parts. In addition, they are only active at high temperaturesand may damage some type of amorphous carbon (DLC) surface.

In addition, from an ecological point of view, it is necessary to reducethe content of sulfur or phosphorus elements of the lubricatingcompositions used.

Thus, organic friction modifiers are studied and conventionally used. Ithas been found that glycerol esters are effective, and, in particular,glycerol mono-oleate is the most commonly used commercially. It has theadvantage of not containing ash, phosphorus or sulfur and being producedfrom renewable raw materials. However, its properties as a frictionmodifier are lower than those of molybdenum dithiocarbamate.

The use of glycerol ethers as a friction modifier is also known. Thus,the application JPS5925890 describes the use of glycerol etherscomprising an alkyl chain comprising from 4 to 28 carbon atoms.JP2000273481 also discloses the use of glycerol ethers comprising analkyl chain comprising more than 14 carbons as friction modifiers.

There is therefore an interest in proposing new friction modifiers toimprove efficiency.

Thus, an object of the present invention is to provide a frictionmodifier and a lubricating composition comprising this friction modifierthat overcome all or some of the aforementioned drawbacks.

Another object of the present invention is to provide an organicfriction modifier derived from renewable raw materials and which doesnot have the drawbacks of MoDTC and which is effective at lowtemperature.

Yet another object of the present invention is to provide a lubricatingcomposition for reducing friction and thus reducing the fuel consumptionof the engine, preferably of a motor vehicle.

The present invention thus relates to a lubricating compositioncomprising:

-   -   at least one base oil; and    -   at least one glycerol mono-ether, characterized in that one of        the alcohol functions of glycerol forms an ether function with a        linear or branched alkyl or alkylene group R comprising from 5        to 8 carbon atoms, preferably from 6 to 7 carbon atoms,        preferably 6 carbon atoms.

Preferably, in the composition according to the invention, the group Rcomprises 6 or 7 carbon atoms.

Preferably, the composition according to the invention is an enginelubricant.

Advantageously, the R group of the glycerol monoether according to theinvention forms an ether bond with any oxygen atom of glycerol, whetherit is bonded to one of the two primary carbons or to the secondarycarbon of glycerol.

Advantageously according to the invention, the glycerol mono-ether ischosen from glycerol monoethers of formula (I) or (II), taken alone oras a mixture:

in which R represents a linear or branched alkyl or alkylene groupcomprising from 5 to 8 carbon atoms, preferably from 6 to 7 carbonatoms, preferably 6 carbon atoms.

Preferably, in the compounds of formula (I) or (II), R represents alinear or branched alkyl or alkylene group comprising 6 or 7 carbonatoms.

Preferably, the glycerol mono-ether according to the invention is chosenfrom the compounds of formula (I).

Preferably, the glycerol mono-ether according to the invention is chosenfrom the compounds of formula (II).

Preferably, the glycerol mono-ether according to the invention is amixture of at least one compound of formula (I) and at least onecompound of formula (II).

Advantageously, the lubricating composition according to the inventioncomprises from 0.01% to 5% by weight of glycerol mono-ether according tothe invention relative to the total weight of the lubricatingcomposition. Preferably, the lubricating composition comprises from0.01% to 2% by weight, preferably from 0.1% to 1.5% by weight ofglycerol mono-ether as defined above, relative to the total weight ofthe lubricating composition.

The glycerol monoethers according to the invention may be obtained byany technique known to those skilled in the art. In particular, apreferred synthetic route for the preparation of glycerol monoethers ofthe invention consists in the reaction between glycerol and an alcoholROH, wherein R has the definition given above.

The etherification reaction between glycerol and an alcohol ispreferably carried out in the presence of an acid catalyst, preferablyan acid heterogeneous catalyst or a homogeneous acid catalyst.

Preferably, the acidic heterogeneous catalysts are chosen from zeolites,resins or oxides of alumina.

Among the resins, may be mentioned perfluorinated polymer resins such asNafion® NR50, cation exchange resins such as Dowex 50wx8, or among ionexchange resins with acidic properties such as Amberlyst® 15 and 36.

Among the zeolites, particular mention may be made of mordenites, ZMS-5zeolites of the MFI type, BEA zeolites or faujasites. Preferably, theacidic heterogeneous catalyst is a zeolite, preferably a mordenite typezeolite, preferably a mordenite with a Si/Al ratio of 11.

Preferably, the homogeneous acidic catalysts are chosen from phosphoricacid, para-toluenesulfonic acid (APTS) and triflic acid. Preferably, theacidic homogeneous catalyst is para-toluenesulfonic acid (APTS).

Advantageously according to the invention and in order to favor thesynthesis of the glycerol mono-ether, the amount of acidic homogeneouscatalyst involved in the reaction is between 1% and 10 mol% relative tothe glycerol content, preferably it is between 1% and 2.5 mol%.Advantageously, the amount of heterogeneous catalyst involved in thereaction is between 1 and 5 mol% relative to the glycerol content,preferably it is 3.5 mol%.

The alcohol/glycerol molar ratio is optimized in order to obtain thebest possible yield of glycerol mono-ether while limiting the secondaryreactions of glycerol oligomer formation. Preferably, thealcohol/glycerol molar ratio is between 1/6 and 3/1, preferably it is1/1.

The etherification reaction may also, in another embodiment, be carriedout from the di-ether resulting from the reaction of the alcohol onitself.

The etherification reaction of glycerol may be carried out in any typeof reactor known to those skilled in the art. Advantageously, it isconducted in an autoclave.

The conditions of temperature, pressure and the etherification reactiontime of the glycerol can be determined in the usual manner by thoseskilled in the art. Advantageously, the reaction is carried out at atemperature of between 80 and 200° C., preferably between 130 and 160°C., for example it may be carried out at 150° C.

Advantageously according to the invention, the reaction is carried outfor at least 5 hours, preferably for 5 to 48 hours. The reaction timemay be 24 hours or 48 hours.

Advantageously according to the invention, the method comprises apurification step. This purification step may comprise the followingsteps: liquid-liquid extraction and/or fractional distillation underreduced pressure. The liquid-liquid extraction is conducted in thepresence of a pair of solvents, which may be determined in the usualmanner by those skilled in the art.

Preferably, the acetonitrile/heptane pair may be used.

Advantageously, at least one liquid-liquid extraction is conducted,preferably at least two. Even more preferably, three liquid-liquidextractions are carried out.

In a preferred manner according to the invention, during thedistillation step, the initial alcohol and the glycerol mono-ether areseparated.

Advantageously, the process according to the invention makes it possibleto selectively obtain glycerol monoethers.

The lubricating composition according to the invention comprises atleast one base oil.

The base oil(s) used in the lubricating compositions according to theinvention may be chosen from a wide range. The base oil of thelubricating composition used according to the invention may, inparticular, be chosen from mineral, synthetic or natural oils,bio-sourced, animal, plant, known to those skilled in the art.

The base oil(s) according to the invention may be oils of mineral orsynthetic origin, chosen from oils of groups I to V according to theclasses defined in the API classification (or their equivalentsaccording to the ATIEL classification) (Table A), alone or in mixtures.

TABLE A Saturated Viscosity index content Sulfur content (VI) Group IMineral oils <90% >0.03% 80 ≤ VI < 120 Group II Hydrocracked ≥90% ≤0.03%80 ≤ VI < 120 oils Group III Hydrocracked ≥90% ≤0.03% ≥120 orhydro-isomerized oils Group IV Polyalphaolefines (PAO) Group V Estersand other bases not included in groups I to IV

The mineral base oils useful according to the invention include alltypes of bases obtained by atmospheric and vacuum distillation of crudeoil, followed by refining operations such as solvent extraction,desalphating, solvent dewaxing, hydrotreatment, hydrocracking,hydroisomerization and hydrofinishing.

Mixtures of synthetic and mineral oils may also be used.

There is generally no limitation on the use of different lubricatingbases for producing the lubricating compositions used according to theinvention, except that they must have properties, in particularviscosity, viscosity index, sulfur, oxidation resistance, adapted foruse for engines or for vehicle transmissions.

The lubricating composition according to the invention may comprise from50 to 99% of at least one base oil, preferably from 60 to 99%,advantageously from 70 to 99% by weight, relative to the total weight oflubricating composition.

The lubricating composition according to the invention may also comprisenumerous additives.

The preferred additives for the lubricating composition used accordingto the invention are chosen from detergents, antiwear additives, extremepressure additives, viscosity index improvers, dispersants,antioxidants, pour point improvers, anti-foam agents, and inorganicfriction modifiers and mixtures thereof.

The lubricating composition according to the invention may also compriseat least one detergent additive.

The detergent additives generally make it possible to reduce theformation of deposits on the surface of the metal parts by dissolvingthe secondary oxidation and combustion products.

The detergent additives that may be used in the lubricating compositionaccording to the invention are generally known to those skilled in theart. The detergent additives may be anionic compounds comprising a longlipophilic hydrocarbon chain and a hydrophilic head. The associatedcation may be a metal cation of an alkali metal or alkaline earth metal.

The detergent additives are preferably chosen from the alkali metal oralkaline earth metal salts of carboxylic acids, the sulphonates, thesalicylates, the naphthenates and the phenate salts. The alkali andalkaline earth metals are preferably calcium, magnesium, sodium orbarium.

These metal salts generally comprise the metal in stoichiometric amountor in excess, therefore in an amount greater than the stoichiometricamount. These are then overbased detergent additives; the excess metalbringing the overbased character to the detergent additive is thengenerally in the form of an oil-insoluble metal salt, for example acarbonate, a hydroxide, an oxalate, an acetate, a glutamate, preferablya carbonate.

Advantageously, the lubricating composition according to the inventionmay comprise from 2 to 4% by weight of detergent additive relative tothe total mass of the lubricating composition.

Anti-wear additives and extreme pressure additives protect frictionsurfaces by forming a protective film adsorbed on these surfaces.

There is a wide variety of anti-wear additives. In a preferred mannerfor the lubricating composition according to the invention, theanti-wear additives are chosen from phosphosulfur additives such asmetal alkylthiophosphates, in particular zinc alkylthiophosphates, andmore specifically zinc dialkyldithiophosphates or ZnDTPs. The preferredcompounds have the formula Zn((SP(S)(OR¹¹)(OR¹² ₂)), in which R¹¹ andR¹², which may be identical or different, independently represent analkyl group, preferably an alkyl group comprising from 1 to 18 carbonatoms.

Amine phosphates are also anti-wear additives which may be used in thelubricating composition according to the invention. However, thephosphorus provided by these additives may act as a poison of thecatalytic systems of automobiles because these additives are ashgenerators. These effects may be minimized by partially substitutingamine phosphates with non-phosphorus additives, such as, for example,polysulfides, especially sulfur-containing olefins. Advantageously, thelubricating composition according to the invention may comprise from0.01 to 6% by weight, preferably from 0.05 to 4% by weight, morepreferably from 0.1 to 2% by weight relative to the total weight of thelubricating composition, anti-wear additives and extreme pressureadditives.

The lubricating composition of the present invention may also compriseat least one viscosity index improving additive. As examples ofadditives improving the viscosity index, mention may be made ofpolymeric esters, homopolymers or copolymers, hydrogenated ornon-hydrogenated, of styrene, butadiene and isoprene, in particularpolyacrylates, polymethacrylates (PMA) or alternatively olefincopolymers, especially ethylene/propylene copolymers.

Advantageously, the lubricating composition according to the inventionmay also comprise at least one dispersing agent.

The dispersing agent may be chosen from Mannich bases, succinimides andtheir derivatives.

Also advantageously, the lubricating composition according to theinvention may comprise from 0.2 to 10% by weight of dispersing agentrelative to the total mass of the lubricating composition.

Advantageously, the lubricating composition according to the inventionmay comprise at least one antioxidant additive.

The antioxidant additive generally serves to retard the degradation ofthe lubricating composition in service. This degradation may notablyresult in the formation of deposits, the presence of sludge or anincrease in the viscosity of the lubricating composition.

Antioxidant additives act in particular as radical inhibitors ordestroyers of hydroperoxides. Among the antioxidant additives commonlyused, mention may be made of antioxidant additives of the phenolic type,antioxidant additives of the amine type, antioxidant phosphosulfuradditives. Some of these antioxidant additives, for examplephosphosulfur antioxidant additives, may be ash generators. Phenolicantioxidant additives may be ash-free or may be in the form of neutralor basic metal salts. The antioxidant additives may especially be chosenfrom sterically hindered phenols, sterically hindered phenol esters, andsterically hindered phenols comprising a thioether bridge,diphenylamines, diphenylamines substituted with at least one C₁-C₁₂alkyl group, and N,N′-dialkyl-aryl diamines and mixtures thereof.

Preferably, according to the invention, the sterically hindered phenolsare chosen from compounds comprising a phenol group in which at leastone vicinal carbon of the carbon bearing the alcohol function issubstituted by at least one C₁-C₁₀ alkyl group, preferably a C₁-C₆alkylgroup, preferably a C₄ alkyl group, preferably by the ter-butyl group.

Amino compounds are another class of antioxidant additives that may beused, optionally in combination with phenolic antioxidant additives.Examples of amine compounds are aromatic amines, for example aromaticamines of formula NR¹³R¹⁴R¹⁵ in which R¹³ represents an optionallysubstituted aliphatic or aromatic group, R¹⁴ represents an optionallysubstituted aromatic group, R¹⁵ represents a hydrogen atom, an alkylgroup, an aryl group or a group of formula R¹⁶S(O)₂R¹⁷ in which R¹⁶represents an alkylene group or an alkenylene group, R¹⁷ represents analkyl group, an alkenyl group or an aryl group and z represents 0, 1 or2.

Sulfurized alkyl phenols or their alkali and alkaline earth metal saltsmay also be used as antioxidant additives.

Another class of antioxidant additives is copper compounds, for examplecopper thio- or dithio-phosphates, copper and carboxylic acid salts,dithiocarbamates, sulphonates, phenates, copper acetylacetonates. Coppersalts I and II, succinic acid or anhydride salts may also be used.

The lubricating composition according to the invention may contain alltypes of antioxidant additives known to those skilled in the art.

Advantageously, the lubricating composition comprises at least oneash-free antioxidant additive.

Also advantageously, the lubricating composition according to theinvention comprises from 0.5 to 2% by weight relative to the totalweight of the composition, of at least one antioxidant additive.

Also advantageously, the lubricating composition according to theinvention may also comprise at least one pour point depressant additive.

By slowing the formation of paraffin crystals, pour point depressantadditives generally improve the cold behavior of the lubricatingcomposition according to the invention.

As examples of pour point depressant additives, mention may be made ofalkyl polymethacrylates, polyacrylates, polyarylamides,polyalkylphenols, polyalkylnaphthalenes and alkylated polystyrenes.

The lubricating composition may also comprise an anti-foam additivechosen from silicones and their derivatives, such as polysiloxanes andtheir derivatives. Such an anti-foam additive may be Bluesil 47V12500®marketed by Bluestar Silicones. The antifoam additive of the lubricatingcomposition according to the invention may also be chosen from acrylics,such as PC1244® marketed by Mosanto.

Advantageously, the lubricating composition according to the inventionmay comprise at least one additional friction-modifying additive, suchas an inorganic friction modifier. The inorganic friction modifieradditive may be selected from a compound providing metal elements and anash free compound. Among the compounds providing metal elements, mentionmay be made of transition metal complexes such as Mo, Sb, Sn, Fe, Cu andZn, the ligands of which may be hydrocarbon compounds comprising oxygen,nitrogen, sulfur or phosphorus.

In particular, mention may be made of organomolybdenum compounds whichrepresent the most important family of friction modifiers. Molybdenumdithiocarbamate (MoDTC) is the best known compound in this family.

Advantageously, the lubricating composition according to the inventionmay comprise from 0.01 to 5% by weight or from 0.01 to 2% by weight,preferably from 0.1 to 1.5% by weight or 0.1 at 2% by weight relative tothe total weight of the lubricating composition, of the additionalfriction modifier additive.

The invention also relates to the use of a lubricating compositionaccording to the invention for lubricating an engine, preferably a motorvehicle engine.

The invention also relates to the use of a lubricating compositionaccording to the invention for reducing the friction in an engine,preferably a motor vehicle engine.

The invention also relates to the use of the lubricating compositionaccording to the invention for reducing the fuel consumption of anengine, preferably of a motor vehicle engine.

Preferably, the reduction of friction in an engine or the reduction ofthe fuel consumption of an engine is measured with respect to thefriction or fuel consumption measured with respect to a referencecomposition.

Advantageously, the reference composition does not comprise glycerolmono-ether, characterized in that one of the alcohol functions ofglycerol forms an ether function with an alkyl or alkylene R groupcomprising 6 or 7 carbon atoms.

Advantageously, the invention also relates to the use of the lubricatingcomposition according to the invention for reducing the fuel consumptionat the start and during the operating phase of the engine.

The invention also relates to the use of a glycerol mono-ether asdefined above in a lubricating composition for reducing the friction inan engine or for reducing the fuel consumption of an engine, preferablya motor vehicle engine.

Preferably, the use of a glycerol mono-ether of formula according to theinvention as defined above allows a reduction in fuel consumption atstartup and during the operating phase of the engine.

The invention also relates to a method of lubricating an engine,preferably a motor vehicle engine comprising the implementation of alubricating composition according to the invention.

The invention also relates to a method for reducing the fuel consumptionof an engine, preferably a motor vehicle engine comprising theimplementation in the engine of a lubricating composition according tothe invention.

The various aspects of the invention are illustrated by the followingexamples, but are not limited thereto.

EXAMPLES

Compounds Used:

The different glycerol monoethers tested are described in Table I below.The synthetic route requires that the glycerol and fatty alcohol used issoluble one in the other. However, any fatty alcohol comprising 8 ormore carbon atoms is not soluble in glycerol, wherein the correspondingglycerol ethers can not be synthesized according to the describedsynthetic route.

TABLE I Name Formula MEG C7P

MEG C7S

MEG C6P

MEG C5P

MEG C4P

MEG C3P

The operating conditions used for the preparation of each of the aboveglycerol monoethers are presented in the following Table II:

TABLE II Efficiency in Name Reactor Catalyst Temperature Period MEG (%)MEG B 2.5% mol 160° C. 24 h 15 C7P APTS MEG ROR 2.5% mol 150° C. 14 h 10C7S APTS MEG R2L 1% mol 150° C. 24 h 25 C6P APTS MEG ROR 2.5% mol 150°C.  7 h 33 C5P APTS MEG MP 1% mol 150° C. 24 h 15 C4P APTS MEG ROR 2.5%mol 140° C.  5 h 8 C3P APTS

The reactors used are the following:

-   -   B: Buchi glass reactor immersed in an oil bath,    -   ROR: Fast opening reactor with a stirring propeller of 220 mL,    -   MP: Multi-reactors consisting of six autoclaves of 75 mL,    -   R2L: 2L stainless steel reactor with stirring blade.

The amount of catalyst is a molar amount relative to the amount ofglycerol used.

For the synthesis of each of the above glycerol monoethers:

-   -   the glycerol, the catalyst and the alcohol are mixed and react        together;    -   then the mixture thus obtained is first purified by decantation        without solvent, then subjected to a liquid-liquid extraction        and finally subjected to fractional distillation to recover the        glycerol mono-ether.

The amount of glycerol monoethers obtained at the end of the reaction ismeasured by gas chromatography.

TGA Measurements

Thermogravimetric analysis (TGA) gives the mass loss of a sample as afunction of temperature. The sample is heated according to programmedtemperature ramps. The mass losses are determined using a scale thatcontinuously measures the mass of the sample during its rise intemperature. The results of these TGA measurements on the variousglycerol monoethers above are described in Table III.

TABLE III % of weight loss Temperature (° C.) MEG C3P 100% 169.5° C. MEGC4P 100% 186.6° C. MEG C5P 100% 192.4° C. MEG C6P 100% 216.1° C. MEG C7P100% 222.9° C. MEG C7S 100% 195.1° C.

Thus short glycerol monoethers (MEG C3P and MEG C4P) have a loss of massof 100% for lower temperatures, and especially less than 190° C. whichmakes them difficult to use in an engine application or evenincompatible with such an application.

Conversely, the glycerol monoethers according to the invention (MEG CSP,MEG C6P, MEG C7P and MEG C7S) have a loss of mass of 100% for highertemperatures, and, in particular, may be greater than 200° C. whichmakes them fully compatible with use in a lubricating composition for aengine application.

Characterization Tests

In order to characterize the behavior of the compositions according tothe invention, two series of tests were performed.

The HFRR (High Frequency Reciprocating Rig) test is used to evaluate theperformance of fuels and lubricants in terms of friction in themixed/limit regime.

This test involves fixing a steel plane, on which the lubricatingcomposition to be tested is placed, in a heating block. A steel ballsubjected to a certain load is then brought into contact with thelubricating composition and the steel plane before being vibrated. Thevibration frequency and the load applied to the ball as well as thetemperature at which the lubricating composition is subjected areadjustable. This test makes it possible to obtain a coefficient offriction curve as a function of the duration of the test.

In practice, the test lasts 30 min, the ball travels back and forth 2 mmat a frequency of 20 Hz, an average shear rate of 40 mm.s⁻¹ (limitregime). At the stop point of the ball, the speed is zero which placesthis test well in a limited and mixed lubrication regime. A load of 200g is applied on the ball, which corresponds to a pressure of 800 MPa.The oil is heated to 100° C.

The MTM test (Traction Machine or Mini Traction Machine) makes itpossible to evaluate the performance of lubricants in terms of frictionin the mixed/hydrodynamic regime. This test consists in setting inrelative motion a steel ball and a steel plane, at different speeds,allowing definition of the % SSR (ratio of the speed of slip/drive speedor Slide-to-Roll Ratio) which corresponds at the slip speed/drive speed.

The tests consist of a 120 min accumulation period where the ballrotates at 100 mm.s⁻¹ (hydrodynamic regime) with a % SSR of 50%, for aload of 1.1 GPa and an oil temperature 100° C.

A friction modifier is considered effective if it lowers the coefficientof friction. The different glycerol monoethers are tested in a referencelubricating composition whose composition is given in Table IV below.

TABLE IV Description Content (%) Base oil 1 Group III (kinematic 59.4viscosity measured at 100° C. according to ASTM D556 = 4 mm²/s), Baseoil 2 Group IV (kinematic 20 viscosity measured at 100° C. according toASTM D556 = 6 mm²/s) Viscosity modifier Poly(isoprene-styrene- 6.6hydrogenated) Pour point depressant polymer poly (alkylmethacrylate) 0.2Antioxidant Diphenylamine 1.5 Additive package P6003 ® marketed by the12.3 company Infineum

Each glycerol mono-ether is added at a content of 1% by weight relativeto the total weight of the reference lubricating composition. Thevarious lubricating compositions tested are described in Table V below.

TABLE V Composition 1 Composition 2 Composition 3 Composition 4Composition 5 Composition 6 Composition 7 Reference 99 99 99 99 99 99 99composition MEG C3P 1 MEG C4P 1 MEG C5P 1 MEG C6P 1 MEG C7P 1 MEG C7S 1Glycerol 1 mono- oleate

Example 1 HFRR Tests

Table VI below represents the coefficients of friction of the referencecomposition and compositions 3, 4, 5 and 6; the values were taken after900 seconds of testing.

TABLE VI Reference lubricating composition Composition 3 Composition 4Composition 5 Composition 6 Coefficient 0.131 0.112 0.119 0.119 0.119 offriction at 900 seconds

Thus, all of the lubricating compositions according to the invention(compositions 3 to 6) significantly lower the coefficient of friction onthis HFRR test under severe conditions.

Example 2 MTM Tests

As previously described, the MTM tests will probe the performance of thefriction modifiers under less severe conditions than the HFRR test butare nevertheless representative of operating points of certain drivingmembers. Table VII below indicates the values of the coefficients offriction of compositions 1, 2, 4, 5, 6 and 7; the values were takenafter 6120 seconds of testing.

TABLE VII Coefficient of friction Composition 1 0.063 Composition 20.045 Composition 4 0.041 Composition 5 0.039 Composition 6 0.040Composition 7 0.045

Thus, the lubricating compositions 4 to 6 according to the inventionsignificantly lower the coefficient of friction on this MTM test incomparison with the comparative compositions 1 and 2 (comprisingrespectively a C₃ and C₄ glycerol mono-ether) or comparative composition7 (comprising glycerol mono-oleate)).

Example 3 Engine Tests

In order to confirm the good results expressed on tribological tests,the composition 3 and the composition 6 were evaluated comparatively bythe implementation of the motor test described below:

During the test, the fluid temperature (water/oil) is regulated. Theengine is then positioned at a selected operating point (rpm/torque) andthe fuel consumption is then measured at this point. A consumption mapis thus produced by this means. The test engine is a Renault R9M engine.

Table VIII shows the gains in terms of % fuel consumption compared tothe reference composition at a temperature (water/oil) set at 90° C. Thethreshold of significance of the test is equal to 0.15%.

TABLE VIII Regime Couple Composition 6 Composition 3 2500 160 0.14 0.312000 160 0.24 0.44 1500 160 0.19 0.35 2500 90 0.37 0.61 2000 90 0.060.50 1500 90 0.28 0.60 2500 40 0.42 0.88 2000 40 0.58 0.82 1500 40 0.710.79 849 2 1.96 0.95 Average heat 0.49 0.62 gain

Thus the lubricating compositions according to the inventionsignificantly improve the fuel consumption, especially when hot.

1-9. (canceled)
 10. A lubricating composition comprising: at least onebase oil; and at least one glycerol mono-ether, characterized in thatone of the alcohol functions of glycerol forms an ether function with alinear or branched alkyl or alkylene group R comprising 6 or 7 carbonatoms, the composition being an engine lubricant.
 11. The lubricatingcomposition of claim 10 comprising: at least one base oil; and at leastone glycerol mono-ether of formula (I) or (II), taken alone or as amixture

in which R represents a linear or branched alkyl or alkylene groupcomprising 6 or 7 carbon atoms.
 12. The lubricating compositionaccording to claim 10 wherein R represents a linear or branched alkyl oralkylene group comprising 6 carbon atoms.
 13. The lubricatingcomposition according to claim 10 comprising from 0.01% to 5% by weight,of glycerol mono-ether.
 14. A method of lubricating an engine comprisingthe implementation of a lubricating composition according to claim 10.15. The method according to claim 14 wherein the engine is a motorengine.
 16. The method for reducing the friction in an engine comprisingthe implementation of a lubricating composition according to claim 10.17. The method for reducing the fuel consumption of an engine comprisingthe implementation of a lubricating composition according to claim 10.18. The method for reducing friction in an engine comprising theimplementation of a glycerol mono-ether as defined in claim 10 in alubricating composition of an engine.
 19. The method for reducing thefuel consumption of an engine comprising the implementation of aglycerol mono-ether as defined in claim 10 in a lubricating compositionof an engine.
 20. The lubricating composition according to claim 11comprising from 0.01 to 2% by weight of glycerol mono-ether of formula(I) or (II) taken alone or as a mixture.
 21. The lubricating compositionaccording to claim 10, comprising from 0.1 to 1.5% by weight of glycerolmono-ether characterized in that one of the alcohol functions ofglycerol forms an ether function with a linear or branched alkyl oralkylene group R, wherein R represents a linear or branched alkyl oralkylene group comprising 6 carbon atoms.